Many early computers aimed at the scientific market had a 36-bitword
length. This word length was just long enough to represent
positive and negative integers to an accuracy of ten decimal digits
(35 bits would have been the minimum). It also allowed the storage
of six alphanumeric characters encoded in a six-bitcharacter
encoding. Prior to the introduction of computers, the state of
the art in precision scientific and engineering calculation was the
ten-digit, electrically powered, mechanical calculator, such as those manufactured by Friden, Marchant and Monroe. These calculators had
a column of keys for each digit and operators were trained to use
all their fingers when entering numbers, so while some specialized
calculators had more columns, ten was a practical limit. Computers,
as the new competitor, had to match that accuracy. Decimal
computers sold in that era, such as the IBM 650 and the IBM 7070, had a word length of ten digits, as
did ENIAC, one of the earliest
computers.

These computers used 18-bit word addressing,
not byte
addressing, giving an address space of 218 36-bit
words, approximately 1 megabyte of storage. Many of them were
originally limited to a similar amount of physical memory as well.
Architectures that survived evolved over time to support larger
virtual address spaces using memory segmentation or other
mechanisms.

Characters were extracted from words either using standard shift
and mask operations or with special-purpose hardware supporting
6-bit, 9-bit, or variable-length characters. The Univac 1100/2200
used the partial word designator of the instruction or a
"J" register to access characters. The GE-600 used special indirect
words to access 6- and 9-bit characters; the PDP-6/10 had special
instructions to access arbitrary-length byte fields. The C
programming language requires that all memory be accessible as
bytes, so C implementations on
36-bit machines use 9-bit bytes.

By the time IBM introduced System/360, scientific
calculations had shifted to floating point and mechanical
calculators were no longer a competitor. The 360s also included
instructions for variable length decimal arithmetic for commercial
applications, so the practice of using word lengths that were a
power of two quickly became universal.